This invention relates, in general, to radio navigation receivers, and more specifically, to a rapid acquisition GPS (Global Positioning System) receiver.
Radio navigation systems are used in tracking aircraft, boats, and land vehicles such as trucks and emergency vehicles. To avoid the limitations inherent in navigation systems utilizing terrestrial transmitters, the GPS has been developed and maintained by the U.S. Government. The GPS uses a network of satellites which can be accessed anywhere in the range of the orbiting satellites. An explanation of the operation of the GPS, as well as a history of the receivers designed to operate with the GPS, is found in U.S. Pat. No. 4,785,463 issued Nov. 15, 1988 to Robert V. Janc and Steven C. Jasper, and U.S. Pat. No. 4,701,934 issued Oct. 20, 1987 to Steven C. Jasper. Both patents are assigned to the same assignee as the present invention.
Rapid acquisition of GPS signals is often more important than accuracy of the information received, particularly in an environment where signal dropout due to interference is likely. Current techniques for signal acquisition, such as those described in the above referenced patents, require several seconds for acquisition. A major factor distinguishing GPS receivers, particularly small, low cost receivers requiring less power, is the speed of acquisition of the GPS signals. The faster the acquisition rate with reasonable accuracy, the more competitive the receiver is.
Another feature of a receiver which is necessary to ensure competitiveness in the GPS receiver market is the ability of the receiver to track multiple satellites simultaneously. In an effort to track satellites, GPS receivers to date have incorporated either sequential or parallel layout architectures. Advocates of sequential architecture claim that sequential architecture receivers reduce hardware cost and reduce interchannel biases that exist in the measurement of relative code phase between multiple satellites. Sequential receivers multiplex all the hardware between the various satellites to be tracked, permitting each satellite to be tracked for a fraction of the total time in a multiplexing manner. With the advent of high speed sampling, however, parallel architectures have been developed with significant reductions in cost and improvements in tracking performances. In parallel architectures, IF samples are processed digitally at rates in excess of the received code rates. This permits the processing of multiple channels with additional correlation ASIC processors. The sensitivity to receiver clock errors is greatly reduced. However, even with the increased speed of the digitally processing parallel architecture, all GPS receivers to date require several seconds for signal acquisition.